This invention relates generally to methods and apparatus for processing computed tomographic (CT) images, and more particularly to methods and apparatus for improving temporal resolution of CT images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Some applications of CT imaging use gated CT data. For example, in some cardiac imaging applications, projection data is retrospectively gated by a patient EKG signal. With retrospective gating, projection data of each cardiac cycle is divided into multiple bins, each of which covers a small portion of the cardiac cycle. In known gated CT applications, temporal resolution and signal to noise ratio are limited.
For example, a multi-detector row CT (MDCT) imaging system is used with a fast scan pitch of 3:1 to 6:1 and with a high gantry rotation speed Gs in the sub-second range. Exemplary rotation rates of known CT imaging systems include rotation rates of 0.5 and 0.8 sec per rotation. When a half scan reconstruction (HSR) is used for improving temporal resolution of the reconstructed image, the temporal resolution Tr is:                                           1            2                    =                                                    π                +                γ                                            2                ⁢                π                                      ⁢                          G              s                                      ,                            [        1        ]            
where the fan angle of detector arc is xcex3, and the half scan reconstruction uses only data of xcfx80+xcex3 view coverage or gantry rotation to reconstruct an image. The temporal resolution is a function of both fan angle xcex3 and Gs. For example, with Gs=0.5 sec and xcex3=60xc2x0, Txc2xd=330 msec, which may not be sufficient to image the heart at its diastole phase. It would therefore be desirable to collect data from multiple cardiac cycles to improve the temporal resolution and/or signal to noise ratio of the reconstructed image.
There is therefore provided, in one aspect of the present invention, a method for multisector computed tomographic (CT) imaging of a cyclically moving object, including steps of: helically scanning a cyclically moving object with a CT imaging system having multiple detector rows and a rotating gantry, at a gantry rotation speed selected either to lead or to lag a cycle of the cyclically moving object; retrospectively gating the projection data of the object with a gating signal related to the motion of the cyclically moving object so that a geometric phase difference is created between a cycle of the rotating gantry and the motion of the cyclically moving object; and reconstructing an image of the cyclically moving object using gated sector data from image data representing a plurality of consecutive cycles of the cyclically moving object.
Where the cyclically moving object is a patent""s heart, this embodiment allows a non-invasive cardiac CT examination to be achieved in a single breath-hold with either a non-contrast enhanced scan to demonstrate the calcification deposit at various coronary arteries, or a contrast enhanced scan to depict the anatomical structure of the coronary artery and to assess dynamic functions of the heart such as ejection fraction, wall motion and valvular function.
The above-described embodiment of the present invention also provides substantial improvement in resolution because the phase difference or temporal resolution can be made as short as 100 msec to freeze the cardiac motion in diastole. Embodiments of the present invention are particularly useful in patients without arrhythmia.